The present invention relates generally to the field of voltage regulators, and battery charging systems which utilize voltage regulators. Such systems are commonly utilized in automotive applications where a voltage regulator provides field current excitation for an alternator that, in turn, provides charging current for maintaining a battery at a predetermined voltage level.
In such systems, operation is commenced in response to the closure of an ignition switch which results in a battery supplying initial field coil excitation to the alternator. Typically, subsequent field coil excitation and operative power for the voltage regulator are provided by an auxiliary alternator power supply output voltage provided by an alternator auxiliary diode trio and provided in accordance with alternator power output. Examples of such prior systems are illustrated in U.S. Pat. Nos. 4,386,310, 4,388,556 and 4,388,587, assigned to Motorola, Inc.
Typically, the initial field coil excitation current is provided by the ignition switch connecting a battery potential to the field coil through an indicator lamp. The ignition switch also is used to connect this same battery potential to other automotive loads such as an ignition spark timing module, a fuel pump and/or a fuel supply valve such as is used in diesel engines. Because the battery potential supplied by the ignition switch and the auxiliary alternator power supply output voltage are both connected to supply field coil excitation current, some prior systems have encountered the problem that when the ignition switch is opened, the loads which were receiving battery potential now continue to receive DC operative power due to connections to the terminal at which the auxiliary alternator power supply voltage is developed. This can result in engine run-on after the opening of the ignition switch wherein this effect is commonly termed "dieseling".
To correct the above-noted problem, some prior art systems and circuits, such as those shown in FIGS. 1 and 2, have utilized a diesel diode implemented external to an integrated circuit (IC) voltage regulator to prevent the auxiliary alternator power supply voltage from providing DC current to engine loads which are only to be operative in accordance with the opening and closing of the ignition switch. While such systems are feasible, the providing of this additional external diode component with respect to the integrated circuit voltage regulator adds to the assembly cost and expense of a charging system. Simply incorporating such a diode within the integrated circuit is typically not feasible or desirable since the diode must be able to withstand large reverse voltages caused by transient signals produced by the electrical loads powered by the ignition switch directed battery voltage. Also, the providing of a diesel diode, whether inside an IC voltage regulator or external to it, somewhat degrades the performance of prior charging systems since the diesel diode implements a forward biased diode voltage drop of approximately 0.7 volts. This results in reducing the battery-supplied voltage potential that is to be used for developing initial field coil excitation. In other words, while providing a diesel diode may solve the problem of current flow from the auxiliary alternator output to various electrical loads, this reduces the battery potential available for initial field current for the alternator and, therefore, reduces alternator performance during its most critical operation corresponding to engine start-up.